Electrolytic pickling, polishing, or degreasing of steel plates is effected for the removal of oxide layers formed on steel plates after annealing thereof, the removal of undesired materials such as oxides, carbides, silicates, oils or other organic materials, present on steel plates as a pretreatment for plating, or other purposes.
In these steel plate surface treatments, steel plates are immersed in aqueous solutions which are acidic, neutral, or alkaline, and electrolysis is conducted using the steel plates as the anode or cathode while applying direct current or alternating current or both. The removal of undesired surface materials (impurities) such as oxide layers, etc., is accelerated either by the dissolution of the metal at the surface of the steel plate or the generation of oxygen when the steel plate forms the anode or by the generation of hydrogen when the steel plate forms the cathode.
As the electrodes to which current is applied in the electrolytic pickling or polishing, high-silicon cast iron electrodes, i.e., iron-silicon alloy electrodes, have conventionally been used. However, this type of electrodes are defective in that, when used as the anode, iron in the alloys is dissolved away and silicon forms silica which is an electrically insulating material, although such electrodes do not cause particular problem when used as the cathode. For this reason, the voltage increases during the use of such electrodes and, as a result, the electrodes are heated and, hence, distorted and cracked, so that their lives are at the most 3 to 4 months although varied depending on use conditions. Furthermore, silicon or silica is dispersed in the electrolyte and adheres to the steel plate as a silicate in a high pH range to stain the steel plate.
Because of the above, high-silicon cast iron electrodes used must be exchanged frequently for new electrodes and at each time the electrolytic cleaning line must be stopped, resulting in a low production efficiency. In addition, the electrode-exchanging operations are not easy because of the heaviness and fragility of the electrodes. The high-silicon cast iron electrodes also have a problem that they are expensive.
On the other hand, carbon electrodes and graphite electrodes used in electrolytic degreasing, etc., are defective in that they should be made to have a large thickness because their electrical resistance is relatively high and their strength is low, and that these electrodes release carbon particles during electrolysis to contaminate the electrolyte. If such carbon particles adhere to the steel plate, plating of the steel plate results in uneven plating and poor bonding strength of the metal coating. Further, in the case where carbon or graphite electrodes are used as the anode, the electrodes are oxidized by oxygen generated and are consumed with evolution of carbonic acid gas. Accordingly, the carbon or graphite electrodes must also be frequently exchanged for new electrodes and, like the high-silicon cast iron electrodes described above, have the problems such as low production efficiency, difficulty in electrode-exchanging operation, fragility, etc.